In for instance OFDMA systems, before the receiver can decode signals, the receiver needs to establish the given timing properties for the transmitter. The timing properties are dependent on the round trip timing between the transmitter and the receiver. For this purpose, the transmitter emits specific patterns or signatures, such as CDMA codes, to be used in a process step denoted initial ranging. IR, by the receiver. During the initial ranging, a sub-set of non-adjacent sub-carriers are transmitted in parallel with normal traffic on other sub-carriers. By performing initial ranging; parameters such as delay, frequency offset and channel quality for a mobile station can subsequently be established. When the base station has performed initial ranging, it instructs the mobile station to adjust uplink transmissions according to a desired timing regime.
A brief overview over OFDMA systems and especially sub-channel coding properties for multi-cellular use is given in prior art document “Orthogonal frequency division Multiple access: Is it the multiple access system of the future?”, Srikanth S., Kumaran V., Manikandan AU-KBC Research center, Anna University, Chennai, India, downloaded from the internet on 2009-09-30.
In one OFDMA implementation, WiMAX, the OFDMA symbol timing is fixed at the base station and various timing advances are used to align all mobile stations. This means that the base station can send timing adjustment messages to the mobile station, so the mobile station signal is aligned with the base station timing. Time domain samples are transformed to frequency domain, based on the common OFDMA symbol timing.
FIG. 1a shows a block-diagram of a WIMAX OFDMA base station receiver 1 according to an internal reference design of the applicant. A radio signal RF is processed in a radio front end unit, 301. The initial ranging patterns are detected by means of initial ranging chain IRc 308-313, for providing a time reference, TR. This is done separately from the receiver chain RXc, formed by stages 302-307, in which signals for time aligned users are processed for reception, such that respective digital output, DO, signals are generated. The processing in the receiver chain RXc is possible when the time reference signal TR has been established/updated by the initial ranging chain IRc.
Stages 305-307 of the receiver chain RXc is provided for each user (stages for further users not shown) and the processing in these stages is subject to user specific parameters, whereas the processing in stages 308-313 and stages 303-304 is common for all users. The receiver chain comprises a cyclic prefix removal stage 302, a Fast Fourier transformation stage, 303, a SC (sub-carrier) de-randomization stage, 304, a de-mapping stage 305, a burst demodulator, 306, and a burst decoder, 307. The SC de-randomization stage, 304 reorders the sub-carriers that have been pseudo-randomly permutated in the receiver, dictated by the given standard under which the receiver is intended to work. The reordering is basically a frequency-hopping scheme that makes the transmission more robust to frequency selective fading or interference. The burst decoder provides the decoded digital output signal, DO. The initial ranging chain comprises an overlap insertion stage 308, a Fast Fourier stage 309, a matched filtering stage 310, an inverse Fast Fourier stage 311, an overlap removal stage 312, providing a detect signal 46 and a peak detection stage 313, providing the time reference signal TR.
The overlapping performed in stage 308 corresponds to a known method of doing correlation in the frequency domain, whereby the side effects of the cyclic convolution (inherent of the frequency domain method) are avoided.
FIG. 3 illustrates how the WIMAX OFDMA mode IR (Initial Ranging) signal is generated for subsequently being processed using the matched filter as represented by among others stage 310 in FIG. 1a. In this application, the initial ranging (IR) signal is also referred to as signature signal 30.
A sub-set of the available sub-carriers are allocated for IR during a given number of OFDMA symbols, i.e. a given period of time. Each mobile station not yet aligned with the base station may transmit signature signals using these sub-carriers and a specific time slot according to rules specified in the standard and according to parameters communicated by the base station in a periodic broadcasting message. The mobile station uses a CDMA code, selected from a finite set of CDMA codes, to modulate the IR sub-carriers 31, and then uses an iFFT 32 to calculate time domain samples 33. This time domain sample, also denoted basic sequence 33—can be split in shorter sequences, e.g. in 8 parts, 37. These parts are copied such that a resulting signature signal 30 appears which comprises for instance one copy of the basic sequence 34 and one repetition 35 of the basic sequence. Finally, padding parts 49 (in this case 7 and 0) are provided, thus forming the particular recognizable signature signal, 30. The padding parts are selected such that the padding parts and the parts of the sequence are cyclically repeated over the signature signal, e.g. 7 is arranged next to 0. It is noted that for general applications not having regard to the WiMAX OFDMA standard, other signature signals could be envisaged comprising more repetitions or no repetitions of the basic sequence 33. The mobile station transmits this signature signal 30 to call for the attention of the base station.
The signature signal 30 will arrive at the receiving base station delayed, because of the round-trip-time (RTT), which for mobile applications may be varying over time as the terminal may move. As mentioned above, it is crucial that the base station can estimate this delay (RTT) so it can send appropriate alignment messages to assure the RTT is compensated for and the transmissions from the mobile station can be aligned in time when arriving at the base station.
FIG. 3 shows further that the received signature signal 30 is filtered by matched filter 310 in the receiver shown in FIG. 1. In this particular embodiment, the matched filter 310 is based on a filtering sequence 36 that is matched with the basic OFDMA symbol 33 that is used to build the full signature signal 30. The matched filter could be matched with a filtering sequence 36 corresponding to different sub-sequences 43 of the actual signature signal 30, a trade-off being made between the power of the peak and the number of mirror peaks.
FIG. 3a illustrates the response of the matched filter 310 when subject to the signature signal 30. From the position of the resulting peaks 38 provided at the output of the filter 310, the timing properties of the received IR signals can be resolved. Aliases (mirror peaks) 40 are also present in output but are discernable from the peaks 38 due to their smaller amplitude, and predictable positions
It appears that the FIG. 1a solution requires redundant FFT means 303.